Integrated circuits are made in a bulk parallel process by patterning and processing semiconductor wafers. Each wafer contains many identical copies of the same integrated circuit referred to as a “die.” It may be preferable to test the semiconductor wafers before the die is cut into individual integrated circuits and packaged for sale. If defects are detected the defective die can be culled before wasting resources packaging a defective part. The individual dies can also be tested after they have been cut into individual integrated circuits and packaged.
To test a wafer or an individual die—commonly called the device under test or DUT—a probe card is commonly used which comes into contact with the surface of the DUT. The probe card generally contains three unique characteristics: (1) an XY array of individual probes that move in the Z direction to allow contact with the die pad; (2) an electrical interface to connect the card to a circuit test apparatus; and (3) a rigid reference plane defined in such a way that the probe card can be accurately mounted in the proper location. When the probe card is brought in contact with the die pad, the Z-direction movement allows for a solid contact with the probe tip. The probe card ultimately provides an electrical interface that allows a circuit test apparatus to be temporarily connected to the DUT. This method of die testing is extremely efficient because many dies can be tested at the same time. To drive this efficiency even higher, probe card manufacturers are making larger probe cards with an ever-increasing number of probes.
A number of designs and manufacturing technologies have been developed for the probe card manufacturing industry. One such technology, herein referred to as “single probe pick and place,” or “pick and place” for short, has been used for testing semiconductor chips. The typical method of testing using a “pick and place” system involves testing a semiconductor chip on a common holder and using several tens, hundreds and sometimes thousands of individual probes mounted to the holder to test an individual pad of the semiconductor chip. For example, one such holder comprises one or more rigid plates with individual holes or cavities, so arranged to mirror the patterns of the electrical pads on the targeted chip or a group of chips. Probes manufactured for pick-and-place assembly are often called vertical probes, owing the moniker to a probe shank extending substantially perpendicular to the upper surface of the semiconductor chip under test.
U.S. Pat. No. 6,359,454 discloses a pick-and-place mechanism for assembling a large number of contactors on a contact substrate. The pick-and-place mechanism includes a first area for positioning an intermediate plate having a plurality of contactors thereon, a second area for positioning the contact substrate for receiving the contactors thereon, a carrier provided between the first and second areas for converting a direction of the contactor to a predetermined direction when receiving the contactor on a seat having an inclined back wall and a flat bottom surface, a first transfer mechanism for picking the contactor from the intermediate plate and placing the contactor on the seat of the carrier, and a second transfer mechanism for picking the contactor from the seat of the carrier while maintaining the predetermined direction of the contactor defined by the carrier and placing the contactor on the contact substrate.
Pick and Place methodology substantially prescribes that probes be manipulated one at the time. As pad pitch shrinks, several disadvantages to pick and place can be noted. The handling of ever smaller probes can prove difficult. Furthermore, larger probe counts impacts both manufacturing through-put and cost structures, as the assembly time for each probe head is often proportional to the number of probes required by the end application. Shrinking pitch also requires that the real estate dedicated for each electrical pad shrinks as well, making the insertion of each probe even more difficult. Probe-to-probe positional accuracy might also be impacted because shrinking pitch might require machining tolerances (e.g. machining a hole for a probe) to shrink as well. Developments in pulse-laser machining, for example, may address this point.
Yet another limitation comes from the manufacturing of the probe holder itself. At smaller pad pitches, it becomes incrementally more difficult to machine individual holes, with individual dividers or walls, to effectively separate and insulate one probe from the neighboring probe.
Therefore, Pick and Place could benefit from a technology that facilitates the handling of small probes and addresses the manufacturing limitations associated with the fabrication of probe holders while, if possible, cutting down on the number of pick and place operations.
A competing technology to Pick and Place, is “micro-electro-mechanical systems probe technology” (MEMS). MEMS generally refers to a process and sometimes a series of repetitive processes that yield many or all probes necessary to the testing of a chip or set of chips. For the probe card industry, MEMS merely refers to the tools, materials and methods utilized during the manufacturing of the probe(s). MEMS probes are typically fabricated on a common holder. Both the probes and holder are typically assembled along with several other mechanical and electrical parts, not discussed herein, to form a functional probe card. One popular MEMS manufacturing approach is based on the creation of a mold, using photosensitive polymers and photolithographic pattern transfer. In one case, the resulting mold is filled with a metal, using, for example, an electro-deposition process. This process, often followed by an encapsulation step in a temporary filling material and a subsequent lapping step to precisely control the final thickness of the electroplated material, is sometimes repeated ad libitum, thereby creating as many individual slices or layers as needed to complete a probe capable to deliver both mechanical and electrical performances as designed and required. In its most simple form, the probe substantially resembles a miniature diving board. Its “foot” is anchored to the supporting holder. The other extremity, the “tip,” is free to move in space, above the holder, above and below a plane substantially parallel to the upper surface of the semiconductor chip under test. For that reason, MEMS probes are sometimes referred to as horizontal probes, in reference to a vertical probe typically found in a Pick and Place approach.
One advantage of MEMS technologies is that arbitrary numbers of probes of varied geometry can, in theory, be fabricated, all at once, on a common holder. Another aspect of a MEMS approach is that the relative position, both in plane and out of plane, of each probe, and especially the tip, with reference to the semiconductor chip under test can be controlled with extremely high precision, in part thanks to the photolithographic methods employed during fabrication.
Yet, the level of ease, MEMS technologies confer to the manufacturing of probes, is not immune to shrinking pad pitch and ever growing probe-array size. In some cases, shrinking pitch and densely populated probe arrays might reduce the real-estate available for designers and manufacturers to produce individual probes capable of operating both electrically and mechanically to the standards and requirements set for testing the end product. One solution is to build probes that extend vertically, or along an axis orthogonal to the plane defined by the upper surface of the semiconductor chip. Such probes could very well resemble typical “pick and place” vertical probes, i.e. very tall structures, sometimes shaped in a “S” or serpentine fashion. Both aforementioned geometrical attributes are ideal to generate a large amount of vertically-directed force to contact the electrical pad and/or a large amount of vertical motion, to accommodate any source of pad-to-pad non-planarity at the surface of the semiconductor chip. The fabrication of such taller probes using MEMS can, however, become prohibitively complicated and, in some cases, limited to the processing capability of key ingredients or key manufacturing steps. For example, making very tall probes with MEMS requires several steps because of the limitations in the deposition of fine material. Specifically, if the channel (which will become the probe structure) in which fine material is to be deposited is too deep, then the material will not deposit evenly throughout the channel, causes the resulting structure to potentially have voids or weak spots. To remedy this problem, the structure can be constructed in several steps, each building the structure taller. But the problem with this is that the several steps require masking, acid and heat steps, and those steps may be misaligned (causing a non-uniform structure), and the acid/heat steps can cause material fatigue that compromises the integrity of the structure when completed.
The probe head manufacturing industry would benefit from a method that retains some of the most attractive aspects of MEMS, including the possibility to fabricate more than one probe at the time, and combines them with some of the most attractive aspects of “Pick and Place,” especially the possibility to manipulate vertical probes, otherwise difficult to fabricate in-situ using MEMS alone. For example, a tall probe structure could be constructed horizontally using MEMS (therefore eliminating a problem with fine material deposition) and then pick and placed on a substrate in the vertical position. Such methods exist and have been employed with success to fabricate probe arrays for pitch size as small as ninety micrometers.
U.S. Pat. No. 6,466,043 discloses a contact structure for testing a semiconductor wafer, a packaged LSI or a printed circuit board that is formed on a planar surface of a substrate by aforementioned photolithography technology. The contact structure is formed of a silicon base having an inclined support portion created through an anisotropic etching process, an insulation layer formed on the silicon base and projected from the inclined support, and a conductive layer made of conductive material formed on the insulation layer so that a contact beam is created by the insulation layer and the conductive layer. The contact beam exhibits a spring force in a transversal direction of the contact beam to establish a contact force when the tip of the beam portion is pressed against a contact target.
U.S. Pat. No. 6,472,890 discloses another method of producing such a contact structure for electrical communication with a contact target. The method includes the steps of providing a silicon substrate cut in a crystal plane, applying a first photolithography process on an upper surface of the silicon substrate for forming an etch stop layer, forming a first insulation layer on the etch stop layer, forming a second insulation layer on a bottom surface of the silicon substrate, applying a second photolithography process on the second insulation layer for forming an etch window, performing an anisotropic etch on the silicon substrate through the etch window for forming a base portion of a contactor, depositing conductive material on the first insulation layer for forming a conductive layer in a beam shape projected from the base portion, and mounting a plurality of contactors produced in the foregoing steps on a contact substrate in predetermined diagonal directions.
U.S. Pat. No. 6,420,884 discloses another contact structure for testing a semiconductor wafer, a packaged LSI or a printed circuit board that is formed of contact beams and a contact substrate. The contact beam is configured by a silicon base having an inclined support portion created through an anisotropic etching process, an insulation layer having a planar shape and formed on the silicon base and projected from the inclined support, and a conductive layer having a planar shape and made of conductive material formed on one surface of the insulation layer so that a beam portion is created by the insulation layer and the conductive layer. The insulation layer and the conductive layer have substantially the same length. The beam portion exhibits a spring force in a transversal direction of the beam portion to establish a contact force when the tip of the beam portion is pressed against a contact target.
U.S. Pat. No. 6,232,669 discloses another contact structure for establishing electrical communication with contact targets with improved contact performance including frequency bandwidth, contact pitch, reliability and cost. The contact structure is formed of a plurality of finger like contactors mounted on a contact substrate. Each of the contactors includes a silicon base having an inclined support portion, an insulation layer formed on the silicon base and projected from the inclined support, and a conductive layer made of conductive material formed on the insulation layer so that a beam portion is created by the insulation layer and the conductive layer, wherein the beam portion exhibits a spring force in a transversal direction of the beam portion to establish a contact force when the tip of the beam portion is pressed against a contact target. An adhesive is applied for bonding the contactors to the surface of the contact substrate.
U.S. Pat. No. 6,535,003 discloses another contact structure for electrical connection with a contact target. The contact structure is formed of a contact substrate mounting a plurality of contactors. Each of the contactors is formed of a silicon base having inclined ends, a silicon beam formed on the silicon base having a support end and a contact end, and a conductive layer formed on a top surface of the silicon beam. The support end is slightly projected from the silicon base and the contact end is substantially projected from the silicon base. The contactor is mounted on the contact substrate such that the silicon base and the support end are connected to the surface of the contact substrate through an adhesive, thereby orienting the silicon beam in a predetermined diagonal direction.
U.S. Pat. No. 7,764,152 discloses a probe card having a plurality of silicon finger contactors contacting pads provided on a tested semiconductor wafer and a probe board mounting the plurality of silicon finger contactors on its surface, wherein each silicon finger contactor has a base part on which a step difference is formed, a support part with a rear end side provided at the base part and with a front end side sticking out from the base part, and a conductive part formed on the surface of the support part, each silicon finger contactor mounted on the probe board so that an angle part of the step difference formed on the base part contacts the surface of the probe board.
U.S. Pat. No. 8,097,475 discloses a probe card having a plurality of silicon finger contactors contacting pads provided on a tested semiconductor wafer and a probe board mounting the plurality of silicon finger contactors on its surface, wherein each silicon finger contactor has a base part on which a step difference is formed, a support part with a rear end side provided at the base part and with a front end side sticking out from the base part, and a conductive part formed on the surface of the support part, each silicon finger contactor mounted on the probe board so that an angle part of the step difference formed on the base part contacts the surface of the probe board.
U.S. Pat. No. 8,241,929 discloses a contactor and an associated contact structure, probe card and test apparatus. The contactor may include a base part having three or more steps in a stairway state, a support part with a rear end side provided at the base part and a front end side sticking out from the base part, and a conductive part formed on a surface of the support part and electrically contacting a contact of a device under test.
U.S. Pat. No. 8,237,461 discloses a contactor includes conductive parts for electrical connection with input/output terminals of an IC device; beam parts with the conductive part provided on their main surfaces; and a base part supporting the beam parts in a cantilever manner, the base part has a support region supporting the beam parts and mark formation regions at which first positioning marks are provided, and weakened parts relatively weaker in strength than other parts of the base part are provided between the support region and mark formation regions.
The Applicants hereby disclose a probe design, probe fabrication and a probe head assembly method poised to leverage some of the most attractive aspects of both “Pick and Place” and MEMS, while bridging some of their respective short-comings. The disclosed probe design combines at least two MEMS probes into one, thereby potentially reducing by a minimum factor of two the number of probes typically handled using “Pick and Place” designs. Also disclosed is an assembly method, based on a reconfigurable probe holder, compatible with “Pick-and-Place” handling operations, yet capable of yielding high probe count, small pitch probe heads with potentially micron to sub-micron positional accuracy, at the pitch of interest.